As the midterm campaigns came to a close, President Trump made a gambit: If he appealed to white racial resentment—if he fanned the flames of racial fear and panic—perhaps he could stanch GOP bleeding and prevent a Republican wipeout. At rallies, he warned of immigrant crime and disorder. On Twitter, he portrayed a caravan of Central American migrants as a destructive army of potential terrorists. Concluding this effort, his presidential campaign released one of the most flagrantly...